It is possible to provide a continuous line display of an analog signal with a closed circuit television system by employing a television camera directed at a strip chart recorder displaying the analog signal and superimposing the resultant video signal output on the normal video signal. This approach is cumbersome and difficult to adjust properly.
Another approach, illustrated in FIG. 1, employs an electronic system in which the input is digitized and wherein each level of quantization corresponds to one of the lines of the raster display to give the proper vertical deflection. Any desired number of horizontal sampling position inputs may be stored in shift registers or random access memories, with the readout shifted and new data added on each frame to provide the moving trace display. However, if the input is changing rapidly, successive samples may have a vertical separation of more than one quantization level, or more than 1 line in the display. Therefore, the trace appears to have gaps wherever it has a steep slope.
FIG. 1 shows a simplified form of such a prior art system. During each line of the raster scan, an oscillator and counter, not shown, sequence the address (at 11) to a memory 12 through all the horizontal coordinates of the display, perhaps several hundred, requiring an address of several bits, of the order of 8 bits. Each address results in an output Y.sub.XN of several bits giving in digital form the y coordinate of the signal at that instant of time. This is digitally compared in a comparator 13 to the y coordinate (shown digitally at Y.sub.N) of the line being scanned at the time, and when the two are identical, the comparator 13 provides an output at 14 comprising a brightening signal, which increases the video signal so as to cause a bright spot 15 (see FIG. 2) to be displayed as a point of the trace. FIG. 2 illustrates how gaps appear in the trace where the trace has relatively steep slopes.